Environmental Engineering Reference
In-Depth Information
of Raoult's law, Dalton's law of partial pressures, and
the ideal gas law, which give
for airflow is estimated as 150 darcys, and the radius of
influence of each extraction well is derived from field
measurements as 25 m. If 30% of the air extracted from
each well passes through contaminated soil, estimate
the number of wells required to clean up the spill in
one year.
x p m
RT
=
∑
i
i
i
c
(5.106)
i
where
x
i
is the mole fraction of component
i
in the liquid
phase residual,
p
i
is the pure component vapor pressure
at temperature
T
,
m
i
is the molar mass of component
i
,
R
is the universal gas constant, and
T
is the absolute
temperature. Removal rates,
M
, greater than 1 kg/d are
usually required for SVE systems to be considered
effective (Charbeneau, 2000). The minimum number of
extraction wells required in a SVE system,
N
wells
, can be
estimated by
Solution
The nAPL spilled is a mixture of benzene, toluene,
and
o
-xylene with the following properties (see Appen-
dix B.2):
Saturation
Vapor
Pressure,
p
i
(kPa)
molar
mass,
m
i
(g)
mole
Fraction,
x
i
mass
Fraction
Component
M
fQct
Benzene
0.30
78.1
0.34
6.9
N
wells
=
(5.107)
Toluene
0.60
92.1
0.58
3.7
o
-Xylene
0.10
106.2
0.08
0.9
where
M
is the mass of contaminant spilled and
t
is the
desired cleanup time. Uncertainty in the distribution of
the intrinsic permeability can lead to substantial uncer-
tainty in the number and location of extraction wells
required for a given cleanup time, as well as uncertainty
in the cleanup time corresponding to a given arrange-
ment of extraction wells (massmann et al., 2000).
However, the flexibility of SVE systems allows for
several adjustments during operation, including the
addition or removal of wells from the system and the
adjustment and redistribution of air withdrawal rates.
For SVE to be an effective remediation strategy, the
bulk concentration of hydrocarbons must typically
exceed 500 mg/kg (Charbeneau, 2000), in which case the
hydrocarbons will probably be present as a nonaqueous
phase.
SVE systems are widely used for remediating soils
contaminated with volatile and semivolatile organic
compounds. This popularity is due, in part, to the low
cost of SVE relative to other available strategies, espe-
cially when contamination occurs relatively deep below
the ground surface (massmann et al., 2000).
From the data given,
T
= 273.15 + 20 = 293.15 K, and
R
= 8.31 J/K·mol. The concentration,
c
, of contaminant
in air flowing through the contaminated soil is given by
Equation (5.106) as
3
x p m
RT
1
8 31 293 15
∑
1
i
i
i
[
c
=
=
( .
0 34 6900 78 1
)(
)(
. )
( .
)(
.
)
i
=
]
+
( .
0 58
)(3700 92 1
)(
. )
+
( .
0 08 900 106 2
)(
)(
. )
=
159
g/m
3
The flow rate,
Q
, of air into each extraction well is
given by Equation (5.104). If the pressure at each well
is maintained at 10 kPa below atmospheric pressure,
and atmospheric pressure,
p
atm
, is taken as 101 kPa,
then the pressure at the well,
p
w
, is calculated as
p
w
= 101 − 10 = 91 kPa. From the data given,
L
= 5 m,
k
a
= 150 darcys = 1.48 × 10
−10
m
2
,
µ
= 0.0182 mPa·s =
1.82 × 10
−5
Pa·s (Appendix B.3, air at 20°C),
r
w
= 150/2 =
75 mm = 0.075 m, and
R
= 25 m. Substituting these
parameters into Equation (5.104) gives
2
k
[(
p
/
p
R r
)
−
1
]
a
atm
w
Q L
=
π
µ
p
EXAMPLE 5.20
w
ln(
/
)
w
1 48 10
1 82 10
.
.
×
×
−
10
)
[(
101 91 1
25 0 075
/
)
2
−
]
A nAPL containing 30% benzene, 60% toluene, and
10%
o
-xylene is to be cleaned up using SVE. Soil
samples indicate that a mass of 10
6
kg has leaked from
a large storage tank on an industrial site. The extraction
wells installed at the site are to have a diameter of
150 mm, intake (screen) lengths of 5 m, and the pres-
sure at the intake is to be held at 10 kPa below atmo-
spheric pressure. The air temperature in the soil is
measured as 20°C, the intrinsic permeability of the soil
3
=
5
π
(
91 10
×
−
5
ln(
/
.
)
=
0 464
.
m /s
3
This analysis assumes that the intrinsic permeability
remains constant at 1.48 × 10
−10
m
2
, while in reality the
intrinsic permeability will increase somewhat due to the
removal of nAPL. Since 30% of the extracted air flows
through contaminated soil,
f
= 0.30, and Equation
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